1. Field of the Invention
The invention relates to an aqueous electrolyte battery and a manufacturing method of an aqueous electrolyte battery.
2. Description of the Related Art
One type of battery that has an aqueous electrolyte is an air battery. An air battery is a battery that uses oxygen as positive-electrode active material, and takes in air from the outside and uses it while discharging. Therefore, the proportion of negative-electrode active material inside the battery case can be larger than it can be with another battery that has positive-electrode active material and negative-electrode active material inside of it. Thus in principle, the dischargeable electrical capacity of the battery is large, while the battery itself is small and lightweight. Also, the oxidation power of the oxygen used as the positive-electrode active material is strong, so the battery electromotive force is relatively high. Furthermore, oxygen is an unlimited resource and is a clean material, so the environmental effects from an air battery are small. In this way, air batteries offer, many advantages and are therefore promising for use as batteries for mobile devices, electric vehicles, hybrid vehicles, and fuel cell vehicles and the like.
For example, the Journal of Power Sources 189 (2009) 371-377 describes a Li-air battery that uses a separator made of a lithium ion-conducting ceramic (Li1+x+yAlxTi2-xSiyP3-yO12 (LATP)) and a lithium ion-conducting polymer (Li3-xPO4-yNy (LiPON)), and that uses a LiCl aqueous solution as an electrolyte. This air battery has excellent performance, with an open circuit voltage of 3.64 V at a room temperature of 25° C., and showing no change in the cell constant even if left for one week. Also, Japanese Patent Application Publication No. 2007-524204 (JP-A-2007-524204) describes an active metal, an electrode structure with an active metal insert, and a battery cell that have an ion-conducting protective structure that has a conductive impermeable layer of active metal (such as lithium) separated from an electrode (i.e., an anode) by a porous separator impregnated with a nonaqueous electrolyte (i.e., anode material). This protective structure protects the cathode side active metal that is arranged on the opposite side of the impermeable layer.
With a metal-air battery that uses an aqueous electrolyte, such as that described in the Journal of Power Sources 189 (2009) 371-377, a gas diffusion electrode that reduces oxygen is used for the positive electrode, metal (such as Li, Zn, or Al) is used for the negative electrode, and an alkaline aqueous solution is used for the electrolyte. In particular, when an aqueous solution Li-air battery discharges, it stores discharge product (Li+ and OH−) in the electrolyte, and when an aqueous solution Li-air battery charges, it consumes the Li+ and the OH− in the electrolyte and produces Li and oxygen. With this type of battery, the discharge product can be accumulated as ions in the solution such that discharge is possible even if the saturated solubility of the ions is exceeded. However, when the saturated solubility is exceeded, the discharge product turns into ion crystals (LiOH) and these are deposited as a solid. Here, when the discharge product is deposited as a solid due to solution equilibrium, crystal nuclei tend to form where the surface energy is high, such as on a solid surface. That is, crystal nuclei preferentially form on the electrode surface and the inside wall of the battery. In particular, if the atmospheric conditions necessary for crystal growth inside the battery are uniform, the formation of crystal nuclei and the deposition of discharge product tend to occur on the surface of the positive electrode (i.e., the air electrode). Also, if the crystallized discharge product covers the electrode surface that serves as the discharge reaction site, the discharge reaction thereafter will be impeded. In particular, with an air battery, if a product solid is deposited in a hole of the positive electrode that serves as a diffusion path, oxygen that is the reactant will no longer be able to be supplied. Also, the battery reaction at the positive electrode takes place at a triphasic interface of gas, liquid, and solid, so once discharge product is deposited on the triphasic interface, the charging reaction may also be impeded. Moreover, this deposition of discharge product may impede electrolyte diffusion within the electrolyte, causing the solution concentration distribution to become disproportionate, such that ion conduction may be impeded. In this way, in a battery in which discharge product can be dissolved in a solvent, and the discharge product is deposited and stored as a solid from solution equilibrium at or above the saturated solubility, it is important to appropriately control the location where discharge product is deposited to prevent deterioration of the battery. This kind of problem is unable to be solved even if the technology described in the Journal of Power Sources 189 (2009) 371-377 is combined with the technology described in JP-A-2007-524204.